heat withdrawn from water heater tank

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I would guess that during those times the inlet water is either unusually cold or there is a spike in flow rate. To investigate, plot the output variables Water Heater Use Side Inlet Temperature and Water Heater Use Side Mass Flow Rate. Then look to what is happening on the demand side of the loop to explain. For example if you have both hot water coils and water use, then this sort of thing could happen when the coils shut down and there is no longer warm water mixing with the cold mains water.